Heretofore, certain devices capable of switching between high and low resistance states have depended for their control upon signals provided by a separate sensing element in the circuit containing the device. In the case of semiconductor switches, it has been the practice to employ a separate temperature responsive control element in the circuit therewith to initiate the switching of the semiconductor switch between its high and low resistance states upon attainment of a given temperature level. The semiconductor switches frequently used are thyristors, such as semiconductor controlled rectifiers (SCR's) or Triacs.
The possibility of using temperature levels to control the switching of SCR's and related devices directly has been known for some time as disclosed in Shockley et al. U.S. Pat. No. 3,079,484, dated Feb. 26, 1963. With the exception of the S. V. Jaskolski et al. U.S. Pat. No. 3,971,056 issued July 20, 1976 and assigned to the assignee of this invention, practical uses for such teachings have not been realized, principally because the development of semiconductor devices has been in the opposite direction, that is, in the direction of preventing intrinsic switching below very high temperatures.
The Jaskolski et al. Patent discloses germanium semiconductor temperature responsive switches which are capable of intrinsically switching between high and low resistance states in response to a predetermined temperature level within the range -55.degree. to +55.degree. C.
Germanium is more sensitive to temperature changes than silicon, due to germanium's lower energy gap (energy region between the valence band and the conduction band). Germanium has a total energy gap of 0.75 eV (electron volt). Its density of intrinsic carriers doubles about every 15.degree. C. Silicon, however, has a total energy gap of 1.12 eV so that its rate of change of density somewhat greater with temperature; but its intrinsic carrier density is smaller than that of germanium by more than a thousand so that it can be used to higher temperatures. Hence, germanium was the preferred choice in selecting a semiconductor material suitable for low temperature intrinsic switching capability. Silicon has heretofore been deemed unsuitable for such application.
Furthermore, germanium and silicon exhibit different intrinsic switching mechanisms. While germanium devices switch primarily because of the electric field in the base region, the phenomena predominantly responsive for the switching action of a four layer silicon device is associated with space charge recombination current (recombining of holes and electrons giving rise to a current flow). The value of the space charge recombination current is a function of the host material, carrier lifetimes, an effective carrier lifetime in the depletion region, impurity profiles and device geometry, as will be more fully described hereinafter.
The inherent temperature dependence of a silicon SCR is manifested in that with increasing temperature, the device leakage current increases and the forward breakover voltage decreases; wherein breakover voltage is defined as that value of applied voltage which causes the device to switch between a high resistance low current "off" state and a low resistance high current "on" state. The switching point of a four layer device is thus variable with device operating temperature, an undesirable result in a high power device dissipating power to produce self-heating thereby causing the switching point to continuously change with power dissipation level.
Since this temperature dependent effect is undesirable, manufacturers have developed techniques to reduce this temperature sensitivity. The approach has been to design a device wherein the temperature dependence of the breakover voltage is not manifested until an unobjectionably large temperature is reached, such as 165.degree. C. The designs typically employ a "shorted-emitter" configuration accompanied by at least one large base region on the order of 4 to 8 mils in width. The shorted-emitter "shorts" out some of the thermally generated leakage current and hence decreases the thermal sensitivity of the device. A base region is made very wide with respect to a diffusion length to reduce the base transport efficiency and hence one of the alphas. Switching action will then occur at higher injection levels as the recombination centers tend toward saturation, thus effecting an increase of the minority carrier lifetime in the space charge region causing an increase in alpha, resulting in switching.